Some conventional book binding apparatuses are configured to accumulate, in order, predetermined number of paper sheets or quires such as printing paper sheets to form a batch of paper, fold the batch of paper into halves, bind the folded portion to form a book, and trim the fore edge of the book, or the fore, top, and bottom edges.
Such book binding apparatuses include various types of detectors and monitoring devices in order to prevent the occurrence of book binding errors such as page missing which is a dropout of a portion of paper sheets constituting a book and page redundancy which is redundancy of paper sheets.
However, it has been impossible to overcome the problem that defective products including such book binding errors are progressed to subsequent processes due to the detection accuracies of such detectors and monitoring devices and operators' inefficient handling.
In order to overcome the problem, a weight detecting device is arranged at a process subsequent to the final book-binding process for measuring the weights of finished books for detecting page missing or page redundancy therein and separating defective products and non-defective products (Japanese Patent No. 3496438).
However, the aforementioned configuration has required a wide space for installing the weight detecting device, thereby causing the problem of complicacy and high cost of the entire book binding system.